Summary:
The ::Get from DB is not augmented with an overload method that takes a PinnableSlice instead of a string. Transactions however are not yet upgraded to use the new API. As a result, transaction users such as MyRocks cannot benefit from it. This patch updates the transactional API with a PinnableSlice overload.
Closes https://github.com/facebook/rocksdb/pull/2736

Differential Revision: D5645770

Pulled By: maysamyabandeh

fbshipit-source-id: f6af520df902f842de1bcf99bed3e8dfc43ad96d